Bug 27071: (follow-up) Get rid of loop

Bug 27071: (follow-up) Get rid of loop

This patch gets rid of a loop on the librarary's groups siblings, and
acknowledges ->get_hold_libraries returns a proper Koha::Libraries
iterator that can be chained with a new query on the passed branchcode.
This way we only fetch  a count from the DB and no loop needs to take
place on the code.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Libraries.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>